1. Field of the Invention
The present invention relates to a liquid crystal projector using a liquid crystal panel, and particularly to a video signal processing apparatus for a liquid crystal projector in which video signals applied to a liquid crystal panel are controlled so as to improve brightness and resolution of a displayed image.
2. Description of Related Art
Generally, a liquid crystal projector uses a liquid crystal panel in which light modulation is controlled according to an applied voltage to display an image on a screen. The liquid crystal projector uses a one-panel system, a two-panel system or a three-panel system.
Referring to FIGS. 1 through 3, conventional liquid crystal projectors adopting the one-panel, two-panel and three-panel systems will be described below. In FIGS. 1 through 3, values in the parentheses of the respective reference numerals represent light transmission coefficients in corresponding components. The light transmission coefficients correspond to those of commercial components which are currently available.
FIG. 1 shows a conventional one-panel liquid crystal projector. When the light from a lamp 11 passes through a hot mirror 12, light excluding a visible ray is intercepted. The visible ray is collected by a focus lens 13 and then the collected visible ray is incident to a polarizing filter 14. Only forty percents of the incident light passes through polarizing filter 14 and is incident to a liquid crystal panel 15 for R, G and B primary color signals. Only ten percents of the incident light of liquid crystal panel 15 passes therethrough and then is incident to polarizing filter 16. Ninety percent of the light incident to polarizing filter 16 passes therethrough and then is projected on a screen via a projection lens 17. As described above, since the one-panel liquid crystal projector has a small number of the components, a compact, low-price and easily assembled product may be manufactured. However, since a color signal liquid crystal panel composed of a combination of R, G and B subpixels has been used, an efficiency of light utilization is reduced to 3.06% to resulting in low image brightness.
FIG. 2 shows an optical arrangement of a conventional three-panel liquid crystal projector. The three-panel liquid crystal projector uses three liquid crystal panels for R, G and B color signals, respectively. When the light from a lamp 21 passes through a hot mirror 22, light excluding a visible ray is intercepted. The visible ray is incident to a dichroic mirror 23. The dichroic mirror 23 separates color components of the incident light. The R component reflected from dichroic mirror 23 passes through a mirror 24 and a polarizing filter 25 and is incident to a liquid crystal panel 26 for an R signal. The G and B components transmitted through dichroic mirror 23 are incident to another dichroic mirror 27. The B component reflected from dichroic mirror 27 passes through a polarizing filter 28 and is incident to a liquid crystal panel 29 for a B signal. The G component transmitted through dichroic mirror 27 passes through a polarizing filter 30 and is incident to a liquid crystal panel 31 for a G signal. The light passing through R signal liquid crystal panel 26 and the light passing through B signal liquid crystal panel 29 are incident to dichroic mirror 34 via polarizing filters 32 and 33, respectively. The dichroic mirror 34 transmits the R component of the incident light and reflects the B component thereof, to transfer the incident components to dichroic mirror 35. The light passing through G signal liquid crystal panel 31 passes through polarizing filter 36 and then is reflected from mirror 37. The light reflected from mirror 37 is incident to dichroic mirror 35. The dichroic mirror 35 for combining the colors transmits the R and B components and reflects the G component, so as to be supplied to a projection lens 38. The projection lens 38 projects the incident light on a screen. As described above, since the three-panel liquid crystal projector uses a liquid crystal panel for each color component, the picture of quality is higher and an the total efficiency of light utilization of 4.23% is higher than those of the one-panel system. Thus, the brightness of the image is slightly increased. However, due to the higher number of the components, the structure is complicated. Also, use of the three liquid crystal panels results in increased costs.
A two-panel liquid crystal projector has been developed to solve the drawbacks of the above-described one-panel and the three-panel system. A conventional two-panel liquid crystal projector is shown in FIG. 3. The two-panel liquid crystal projector uses two liquid crystal panels for a luminance signal and a color signal. In FIG. 3, when the light from lamp 41 passes through hot mirror 42, the light excluding a visible ray is intercepted. The visible ray is divided into an S polarized component and a P polarized component by polarizing filter 43. The S polarized component reflected from polarizing filter 43 is reflected again by mirror 44 and is input to a white/black liquid crystal panel 45. The P polarized component transmitted from polarizing filter 43 is reflected by mirror 46 and is input to a color liquid crystal panel 47. The S polarized component is optically modulated as a white and black image by white/black liquid crystal panel 45 and is incident to polarizing filter 48. The P polarized component is optically modulated as a color image by color liquid crystal panel 47 and is incident to polarizing filter 48. The polarizing filter 48 for combining both polarized components transmits the input white and black image and reflects the color image, so as to be supplied to a projection lens 49. The projection lens 49 supplies the white/black image and the color image to a screen, so as to be overlapped and displayed thereon.
The structure of the white/black liquid crystal panel which is used in the two-panel liquid crystal projector is shown in FIG. 4 and that of the color liquid crystal panel is shown in FIG. 5. The pixel structure of the conventional liquid crystal panel has a delta-type pixel array (a triangular structure) in which an even column and an odd column are offset by half of a pixel. The pixel structure of the white/black liquid crystal panel is a field quincunx (QT) structure in space in view of a sampling operation, having a resolution area such as a frequency spectrum shown in FIG. 6. In the pixel structure of the color liquid crystal panel as shown in FIG. 5, the respective subpixels of R, G and B are offset to each other, and are arranged in a rhombic shape. Here, the subpixels of the color liquid crystal panel also have QT structures in space with respect to the respective R, G and B components. However, the horizontal resolution is decreased.
FIGS. 7A through 7D show sampling points of the signals input to the liquid crystal panels with respect to time. In case of the color liquid crystal panel, sampling of the color signals has a polyphase type in which a sampling period of one color signal R, G or B is moved by a 1/3 of a period to sample another component. Meanwhile, in the case of the white/black liquid crystal panel, sampling of the luminance signal is performed by three times the sampling frequency with respect to the color signal as shown in FIG. 7D. Accordingly, the resolution of the image is three times greater than that of the color liquid crystal panel. Such a two-panel liquid crystal projector advantageously has a the resolution as high as the three-panel system. That is, the one-panel system uses a single liquid crystal panel in which the R, G and B components are separately displayed, with a result that the resolution is lowered. The two-panel system has the nearly same resolution as that of the three-panel system. Among the three types of the liquid crystal projector, the efficiency of light utilization is highest as 7.34% in the case of a luminance signal. Also, the two-panel liquid crystal projector is compact and relatively inexpensive.
A video signal processing circuit for use in such a two-panel liquid crystal projector is disclosed in a Japanese laid-open patent publication No. 03201695 on Sep. 3, 1991, entitled "Projection Display Apparatus." The projection display apparatus includes a white/black liquid crystal display device for modulating a light transmission factor of a first linear polarized component based on a luminance signal separated by a YC separation circuit, and a color liquid crystal display device for modulating a light transmission factor of a second linear polarized component based on the separated color signal. However, the apparatus in the prior art processes the luminance signal and the color signal so that they may be just overlapped on a screen. Thus, an unnatural image is displayed on a screen because the black level on the screen is higher than that of an original signal and the contrast is low.